DIELECTRIC-COATED WAVEGUIDE 



1269 



result is for coat thicknesses which cause the wavelength X = 5.4 mm 

 to be critical with respect to TEoi-TMn coupling: 



5 = 0.002 I = \b — = 43.3 20 logio 



OiOl 



E2 



= -1.29 dh 



= 0.004 = 2X6 



= 2.70 



= - 13.32 dh 



0.006 = 3X6 



= 0.169 



= -25.37 dh. 



The values corresponding to 5 = 0.002 do not satisfy the condition (25). 

 Therefore they cannot be considered as a cjuantitative result but only as 

 an indication that the mode conversion is very high. We conclude from 

 these values that the mode conversion is much too high for a coat thick- 

 ness which makes the beat wavelength between TEoi and TMn equal to 

 the supporting distance or half of it. 



If no other measures can be taken, such as remo\dng the periodicity 

 of the supports or inserting mode filters, the lowest critical frequencies 

 of TEoi-TMii coupling have to be avoided. A dielectric coat must be 



10 



a. 



HI 



a 



LU 



< 



UJ 



U 2 



Z 



z 

 o 

 I- 

 < 1.0 



5 0.8 



5 0.6 



Q 



< 0.4 



(/) 

 (/> 



HI 



z 



y 0.2 



< 



o 

 •J 0.1 



AVERAGE RADIUS OF CURVATURE IN FEET 



10" 



Fig. 7 — Optimum coat thickness and increase in TEoi attenuation of a dielec- 

 tric-coated guide with random deviations from straightness. 2a = 2 inches; X = 54 

 mm; e = 2.5. 



